Conventionally, a developer cartridge is detachably mounted on a developing device that is used in an image forming device. The developer cartridge accommodates developer therein and supplies the developer to the developing device when mounted thereon. One of such conventional developer cartridges includes an inner casing and an outer casing, both of which have a hollow cylindrical shape. The inner casing is formed with an inner opening and the outer casing is formed with an outer opening. The outer casing accommodates the inner casing such that relative rotation can be attained between the inner casing and the outer casing.
In this developer cartridge, the developer is supplied to the developing device through the inner opening and the outer opening which are in coincidence with and in communication with each other as a result of relative rotation between the inner casing and the outer casing. Also, a sealing member is provided between the inner and outer casings and at a position around the inner opening so that the developer cannot leak from a gap between the outer casing and the inner casing.